The present invention relates generally to electric motors for automobile applications and, more particularly, to permanent magnet, brush-type, direct current motors which utilize pulse width modulation for speed control.
A significant challenge of permanent magnet, brush-type, direct current motors is to achieve different speeds of operation. Wound field-type motors generally can have speed controlled by altering the field flux. This is done by changing the current or the number of coil turns in the field winding. Since permanent magnet motors have a constant field flux, they cannot achieve speed control by field flux variation.
Often, permanent magnet motors used in automotive applications require the use of more than one speed, usually requiring a lower speed for general purpose operation and a maximum speed for worst case operation. Typically, multiple speeds have been achieved in permanent magnet motors by adding a resistor in series with the motor, switching out brushes (lap wound motor), adding a third brush (short-out coils) or using external electronic control such as analog devices or pulse width modulation.
U.S. Pat. No. 5,119,466 uses pulse width modulation to control the speed of a permanent magnet dc motor. A speed control circuit is carried by a circuit board disposed within the motor. The circuit board includes a motor control signal converter which receives a motor control signal from a vehicles electronic control unit (ECU) and sends a pulse width modulated signal to a field effect transistor (FET), which is also mounted on the circuit board and joined to a projection of the case of the motor. However, the requirement of the circuit board having a signal converter and FET mounted thereon increases the motor cost and the dissipation of heat created by the FET can be improved.
Another known arrangement provides a FET in a separate housing for modulating power to the motor based on a PWM signal from the ECU. However, this arrangement introduces another component mounted in an already crowded engine compartment.
Accordingly, there is a need to provide a direct current permanent magnet motor which utilizes pulse width modulation for speed control and has a metal oxide semiconductor FET (MOSFET) integrated therewith, without requiring a speed control circuit board within the motor.
An object of the invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is achieved by providing an electric motor including a motor housing defining an internal cavity. A brush card assembly, a commutator and an armature assembly are provided in the cavity. The commutator cooperates with the brush card assembly to supply electric current to the armature assembly. Permanent magnet structure is provided in the housing to generate a magnetic field to cause rotation of the armature assembly. A FET is mounted with respect to the housing and the FET has a gate terminal constructed and arranged to receive a pulse width modulated output directly from an electronic control unit which is remote from the motor so as to enable able the motor to operate at more than one speed.
In accordance with another aspect of the invention, a method of controlling speed of an electric motor is provided. The motor comprises a motor housing defining an internal cavity. A brush card assembly, a commutator and an armature assembly is provided in the cavity. The commutator cooperates with the brush card assembly to supply electric current to the armature assembly. Permanent magnet structure is provided in the housing to generate a magnetic field to cause rotation of the armature assembly. A FET is mounted with respect to the housing so as to be in heat exchange relation therewith. The method includes connecting a drain terminal of the FET to a negative lead of the brush card assembly. A source terminal of the FET is connected to a ground terminal or a negative terminal of the motor. A gate terminal of the FET is connected to a pulse width modulated output of an electronic control unit which is remote from the motor. A pulse width modulated signal is sent directly from the electronic control unit to the FET to control a speed of the motor.
In accordance with yet another aspect of the invention, a method of integrating a FET into a motor includes: providing a direct current motor comprising a motor housing defining an internal cavity; a brush card assembly, a commutator and an armature assembly being disposed in the cavity. The commutator cooperates with the brush card assembly to supply electric current to the armature assembly. Permanent magnet structure in the housing generates a magnetic field to cause rotation of the armature assembly. A connector structure is connected to the motor. The connector structure has a mounting surface. Spring structure is associated with the mounting surface. The FET is placed on the spring structure such that the spring structure is disposed between a surface of the FET and the mounting surface. The FET is electrically connected to the motor. An end cap is secured to the motor to close the cavity, with the spring structure forcing a surface of the FET into contact with a surface of the end cap such that heat generated by the FET is transferred directly to the end cap.
Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification.